Pulsed-laser fabrication of gas-filled hollow Co–Pt nanospheres

We report on nitrogen-filled hollow Co–Pt nanospheres produced via pulsed-laser ablation in ambient nitrogen gas. The resulting nanospheres are characterized by a single-crystalline face-centred cubic Co55±3Pt45±3 shell and a void filled with molecular nitrogen, typically occupying the sphere’s central region. The average diameter of the spheres and the voids is 35 ± 8 and 16 ± 2 nm, respectively. The calculated number density of nitrogen atoms, measured within these voids, is 1.58 ± 0.4 nm−3. The resulting pressure in the voids near ambient temperature (300 K) and at the boiling temperature for the Co–Pt alloy (∼3000 K) is estimated to be 1.9 ± 0.3 and 34.3 ± 9 MPa, respectively. The gas-filled Co–Pt hollow spheres are formed in only one step involving two physical processes. First, after each laser pulse, the vaporized, supersaturated Co–Pt ablated species are condensated in the plume under high pressure and temperature, resulting in nitrogen gas trapping. Between two laser pulses, the pressure and temperature in the plume drop rapidly, the nitrogen-rich liquid nanospheres become thermodynamically unstable and the nitrogen gas bubble starts to expand until the solidification of the nanospheres. The fast solidification of the solid shell prevents further outward diffusion of nitrogen and thus an amount of nitrogen gas is preserved in the void. These nanospheres have the potential in biomedical, magnetic and catalytic applications.